Abstract

Interferometry is the most precise measurement technique known today. It is based on interference and therefore on the wave-like nature of the resources – photons or atoms – in the interferometer. As given by the laws of quantum mechanics the granular, particle-like features of the individually independent atoms or photons are responsible for the precision limit – the shot noise limit. However this “classical” bound is not fundamental and it is the aim of quantum metrology to overcome it by employing quantum correlations – entanglement – among the particles. We report on the realization of spin squeezed states suitable for atom interferometry based on two external modes of a Bose-Einstein condensate. We detect manybody entangled states which allow – in principle – for a precision gain of 35% over the shot noise limit in atom interferometry. We demonstrate a novel non-linear atom interferometer for Bose-Einstein condensates whose linear analog – the Ramsey interferometer – is used for the definition of the time standard. Within the non-linear interferometer we detect a large entangled state of 170 inseparable atoms. A measurement with this interferometer outperforms its ideal linear analog by 15% in phase estimation precision showing directly the feasibility of non-linear atom interferometry with Bose-Einstein condensates beyond “classical” precision limits.